In processing food products on an industrial scale, as in the production of margarine and other liquid or liquefiable edible products processed in the liquid state, it is important that the equipment should be, and remain, as clean as possible, more particularly that the presence of microorganisms in the equipment should be avoided as much as possible. As also disclosed in HLM Lelieveld, Hygienic Design and Test Methods, Journal of the Society of Dairy Technology, Vol. 38, No. 1, January 1985, pages 14-16, a distinction is to be made between "hygienic processing" and "aseptic processing". Hygienic processing means in this connection that the increase in the concentration of microorganisms during processing is relatively small owing to the equipment, while during aseptic processing no increase in the concentration of micro-organisms in the product may occur owing to the equipment. It is important both for hygienic and aseptic processing that the equipment employed should be sterilizable and for aseptic equipment also that it should be bacteria tight.
In the processing of liquefied materials, which optionally contain solid constituents, usually first pumping means are present, which transport continuously the product through the processing unit. In the case of consumer products, such as margarine, soups, mayonnaise, etc. these products are usually fed to second pumping means, shortly before a metering device, which second pumping means acts continuously, semi-continuously or discontinuously, but anyway at a lower throughput than said first pumping means. One of the reasons therefor is that the metering device and the packing machine should never be short of product. However this means that the excess of products flowing to the second pumping means should be compensated for. This is usually done by providing a recirculation line, which can be shut off by means of a valve or other means for closing the recirculation line.
Furthermore in the vicinity of the recirculation line a compensating piston is provided for, which piston allows excess material into the piston chamber. Once the piston has reached its outward position, it activates a switch which in turn opens the valve in the recirculation line. The excess material present in the piston chamber is then removed, and when the piston reaches its inward position, the recirculation line is closed again.
With this system it is however very difficult to maintain hygienic or aseptic conditions as set forth hereinbefore, as it is notoriously difficult to make pistons bacteria-tight.